feat(frontend): add Dashboard page and components for #53

Implement the main dashboard homepage with:
- WelcomeHeader: Personalized greeting with user name
- DashboardQuickStats: Stats cards for projects, agents, issues, approvals
- RecentProjects: Dynamic grid showing 3-6 recent projects
- PendingApprovals: Action-required approvals section
- EmptyState: Onboarding experience for new users
- useDashboard hook: Mock data fetching with React Query

The dashboard serves as the authenticated homepage at /(authenticated)/
and provides quick access to all project management features.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/dashboard/index.ts

@@ -0,0 +1,23 @@
+/**
+ * Dashboard Components
+ *
+ * Exports all dashboard-related components.
+ *
+ * @module components/dashboard
+ * @see Issue #53
+ */
+
+export { Dashboard } from './Dashboard';
+export { WelcomeHeader } from './WelcomeHeader';
+export { DashboardQuickStats } from './DashboardQuickStats';
+export { RecentProjects } from './RecentProjects';
+export { PendingApprovals } from './PendingApprovals';
+export { EmptyState } from './EmptyState';
+
+// Re-export types
+export type { DashboardProps } from './Dashboard';
+export type { WelcomeHeaderProps } from './WelcomeHeader';
+export type { DashboardQuickStatsProps } from './DashboardQuickStats';
+export type { RecentProjectsProps } from './RecentProjects';
+export type { PendingApprovalsProps } from './PendingApprovals';
+export type { EmptyStateProps } from './EmptyState';